It is a mystery

Oh, yes, I'm aware-- I've done historical re-enactment and costuming and such. I intellectually know it's perfectly possible and indeed in the total of human history, the majority of spinning has been of very fine stuff. That doesn't mean that I actually have internalized how on earth one would ever manage to spin something fine, because in my cumulative fifteen minutes of spinning, all I've ever made is wildly uneven chunky thick yarn.